This grant is aimed at discovering the etiology of endemic pemphigus foliaceus (PF), also known as Fogo Selvagem (FS) in rural Brazil. This epidermal-specific autoimmune disease is mediated by pathogenic IgG4 anti-desmoglein 1 (Dsg1) autoantibodies. In 1994, we identified the Amerindian Reservation of Limao Verde (LV), Brazil composed of ~1,400 individuals where the prevalence of FS was ~3%. Importantly, we have followed this population clinically and serologically for the last 18 years, identifying transitions from normal to FS in 12 individuals, and reported that exposure to blood feeding arthropods is a potential risk factor for FS. Moreover, the sera of patients with leishmaniasis, Chagas disease and onchocerciasis, where these insects are involved as carriers of parasites, possess anti-Dsg1 antibodies. Recently, we have shown that IgG4 anti-Dsg1 autoantibodies cross-react with conformational epitopes of the LJM11 yellow protein from the saliva of Lutzomyia longipalpis, the vector of leishmaniasis. The objective of this proposal is to determine how Dsg1 becomes the target of the immune system in FS and to identify the environmental antigen(s) that initiate the anti-self response. Based on previous studies, we hypothesize that salivary antigens from hematophagous insects are the source of sensitizing antigen in FS. The aims of this grant will identify epitopes on Dsg1 and LJM11 that cross-react with IgG4 antibodies from FS patients and test whether these antibodies present in patient sera derive from na?ve B cells that bind foreign or self-antigens or both. In Aim 1 we will determine the conformational epitopes bound by IgG4 anti-Dsg1 polyclonal antibodies from sera and B cells of FS patients on Dsg1(self) and LJM11(sand fly) antigens. Moreover, we shall test if murine anti-LJM11 antibodies, that bind human Dgs1, are pathogenic by passive transfer experiments. In Aim 2 we shall generate IgG4 monoclonal antibodies (scFv) from FS B cells by phage display methods and select Dsg1 and LJM11 specific clones by panning procedures. These scFv antibodies will be tested for pathogenicity and used for more precise epitope mapping. Further, revertant monoclonal antibodies will be generated and tested for reactivity with Dsg1 and LJM11 in an effort to disclose the initial trigger of the IgG4 antibody response in FS. Aim 3 and 4 are concerned with seroepidemiological studies in three cohorts and the population of Limao Verde. We shall continue searching for risk factors that may precipitate the IgG autoimmune response in normal subjects and the IgG4 in FS patients. The transition from normal to disease state will likely be uncovered on the follow up studies of the cohorts. At the completion of these novel studies we shall have a better understanding of how environmental antigens might trigger, in certain genetically predisposed individuals, an autoimmune IgG4 response that leads to skin diseases. In parallel, new epitope-specific immunoassays and potential novel therapeutic interventions may evolve from these studies.